We have been interested in the role of RNA splicing in the biogenesis of mRNA. As a model system a small DNA tumor virus (SV40) was utilized. This viral system, or previously constructed mutants thereof, has been used as a vector to insert heterologous DNA segments in order to identify nucleotide signals involved in the regulation of eukaryotic transcription. Of particular interest was the rescue of a splicing defective SV40 deletion mutant from which the late intron has been precisely removed. This mutant, which produces nonstable late viral RNA was used as a recipient for the insertion of an isolated mouse beta maj globin intron 1 and its flanking sequences. The beta maj intron without 5' and 3' ends of the beta maj globin gene was inserted in both the sense and antisense orientation relative to the late SV40 transcription unit, at a site which differs from the location of the original 16S RNA intron. Cloned recombinants harboring the inserted intron in the sense direction give rise to stable mRNA.